The present invention relates to the radiation generation arts. It particularly relates to high brightness electron sources for producing linear hollow electron beams, spinning hollow electron beams, solid electron beams, and the like, and will be described with particular reference thereto. However, the invention will also find application in other radiation sources.
High brightness electron beams having selected spatial and electrical characteristics are used in a wide range of applications. A gyrotron, for instance, employs an electron gun that produces a spinning hollow electron beam. Klystrons operate with linear hollow electron beams. Other applications call for a high brightness solid electron beam.
Heretofore, injection electron guns have generally been used for such applications. These guns include a thermionic cathode, typically in the form of a porous tungsten pellet that is impregnated with barium oxide and other substances. Upon heating, barium migrates to the cathode surface and reduces the work function of the cathode surface and thus facilitates efficient thermionic emission.
However, a problem arises because barium is lost to the vacuum during operation, which limits operating life of the cathode and can produce barium contamination elsewhere in the vacuum system. Moreover, barium loss increases with cathode temperature, which limits operating temperature and hence brightness of the electron gun. Cathode heating also can produce undesired outgassing and heating of nearby components.
The present invention contemplates an improved apparatus and method.